Six new compounds were isolated from the methanol extract of Arachinoides standishii OHWI. Chemical and physicochemical studies showed their structures to be 1-(2, 4, 6-trimethoxyphenyl)-but-trans-2-en-1-one (I), 5, 7-dimethoxy-2-methylchromanone (II), 2-ethyl-5, 7-dimethoxychromanone (III), 5-hydroxy-dimethoxy-2-methyl-chromanone (IV), 3-β-D-allosyloxy-1-(2-hydroxy-4, 6-dimethoxyphenyl)-butan-1-one (V) and 8-carboxy-6-hydroxy-2, 5-dimethyl-2a, 3, 4, 5-tetrahydroacenaphthene (VI). Compound VI, named ryomenin, is the first sesquiterpene reported to possess a tetrahydroacenaphthene skeleton.